Second arrest over London train bombing

A second man has been arrested over Friday's bombing of a London commuter train that injured 30 people as Britain remains on its highest level of alert with soldiers helping provide security.

The 21-year-old man was detained under Britain's Terrorism Act in the west London suburb of Hounslow on Sunday (AEST), the Metropolitan Police said in a statement.

Police arrested an 18-year-old man in the departure lounge of Dover port earlier on Saturday in what they called a "significant" step and then raided a property in Sunbury, a town near London and about 6km from Hounslow.

The home-made bomb shot flames through a packed train carriage at west London's Parsons Green Tube station during the Friday morning rush hour but apparently failed to detonate fully.

Islamic State claimed responsibility, as it has for other attacks in Britain this year, including two in London and one at a concert by American singer Ariana Grande in Manchester in May.

Home Secretary Amber Rudd said the second arrest indicated it was not a "lone-wolf" attack, but there was no evidence Islamic State was involved.

"It is inevitable that so-called Islamic State, or Daesh, will reach in and try to claim responsibility. We have no evidence to suggest that yet," she told the BBC's Andrew Marr Show.

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"But as this unfolds, and as the police do their investigations, we will make sure that we find out exactly how he was radicalised, if we can."

She added that US President Donald Trump's suggestion that the terrorist behind the attack was "in the sights of Scotland Yard" was "pure speculation".

"It's never helpful to have speculation about an ongoing operation and I would include the President of the United States in that comment."

The bomb struck as passengers were travelling towards the centre of London. Some suffered burns and others were hurt in a stampede to escape. Health officials said none was thought to be in a serious condition.

Prime Minister Theresa May put Britain on its highest security level of "critical" late on Friday, meaning another attack might be imminent. Soldiers and armed police were deployed to strategic locations such as nuclear power plants.

On Saturday, armed police patrolled the streets near government departments in Westminster and guarded Premier League soccer grounds hosting matches.

The last time Britain was put on "critical" alert was after a suicide bomber killed 22 people at the Ariana Grande concert.

On that occasion, the threat level remained at critical for four days while police established whether the bomber had worked alone or with others. Prior to that it had not been triggered since 2007.